26 October 2016

Environment

This Technology Revealed Just How Scarily Fast Antarctica Is Melting

Warming ocean waters have been destabilizing some of the massive ice shelves around Antarctica for years. Now scientists have figured out that some of this ice is melting far more quickly than previously thought, according to a study published Tuesday. That has implications for how much sea levels will rise over the next several decades and centuries.

“The system is no longer in equilibrium,” said Ala Khazendar of NASA’s Jet Propulsion Laboratory, the leader of the study published in the journal Nature Communications. “The melting is happening at such a fast rate that the glaciers cannot replenish what has been lost. The net result is that the glaciers are thinning.

This affects the stability of Antarctic ice in two ways, Khazendar explained, because “ice shelves are the gateways and the gatekeepers of Antarctica.”

As glaciers flow slowly off the edge of the continent and into the sea, they begin to float while still attached to the land in a zone scientists call the “grounding line.” It’s natural for seawater to melt the resulting ice shelves, “or pieces of the ice shelf detach from the front, forming what we know as icebergs,” said Khazendar.

That puts new supplies of freshwater into the ocean. For eons the weight and bulk of the ice at the grounding line tended to slow down the flow of ice entering the ocean, while the formation of new ice far inland on Antarctica kept pace with the losses at the edges of the ice sheet.

But using airborne radar to make direct measurements along many miles of the grounding lines of three West Antarctic glaciers that flow into a body of water called the Amundsen Sea Embayment, Khazendar and his colleagues determined that between 2002 and 2009, the glaciers retreated at the fastest speed ever recorded in West Antarctica, melting from the undersides upward.
Scientists have long tracked extreme melt rates at West Antarctica’s Pine Island and Thwaites glaciers, but the three glaciers analyzed in this study—Smith, Pope, and Kohler glaciers, which flow into the Crosson and Dotson ice shelves—are changing even more rapidly, the researchers found.
“The study and its findings are really important because, although we suspected that the West Antarctic ice sheet was melting from underneath where it is in contact with the ocean, we had no direct evidence,” Neil Glasser, an expert on Antarctic ice sheets based at Aberystwyth University in the United Kingdom, wrote in an email. “This study not only provides that direct evidence but also puts some numbers on the rate of submarine ice melt.”

The findings “would tend to suggest that current estimates of sea level rise from the [West Antarctic Ice Sheet] are too low and they need to be revised upwards,” he added.

Since 2009, the glaciers have continued to lose more mass than they’re gaining but at a slower pace than in the 2002 to 2009 period. “These phenomena are complex,” Khazendar said. “In the past couple years, a couple papers have said that retreat of grounding lines in the Amundsen Sea are irreversible. We’ll need more observations and numerical modeling to investigate that.”

NASA radar planes are supposed to take new readings along the same lines soon, he said, which will allow the researchers to update their estimates of the rate and amount of ice loss in the area.
“If this continues, then the West Antarctic Ice Sheet could become unstable and enter a phase of very rapid recession as it melts back at the grounding line,” Glasser said.

Three neighboring glaciers in West Antarctica—the Smith, Pope, and Kohler glaciers—are melting and retreating at an accelerated pace, scientists have learned. (Photo: NASA JPL)
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